[FIX] im_livechat: Style button to submit review

When a visitor close the livechat and submit a review,
the "Send" button is not styled.

closes odoo/odoo#36487

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/im_livechat/static/src/xml/im_livechat.xml

@@ -17,7 +17,7 @@
             <div class="o_livechat_rating_reason">
                 <textarea id="reason" placeholder="Explain your note"></textarea>
                 <div class="o_livechat_rating_reason_button">
-                    <input type="button" class="o_rating_submit_button" value="Send" />
+                    <input type="button" class="btn btn-primary btn-sm o_rating_submit_button" value="Send" />
                 </div>
             </div>
             <div class="o_livechat_email">